1. Field of the Invention
This invention relates to an injection compression molding method of a spectacle lens, in which the spectacle lens is obtained by injecting, compressing and molding a thermoplastic resin, and a spectacle lens produced by using the injection compression molding method. More specifically, this invention relates to an injection compression molding method of a spectacle lens, being capable of molding a plus lens and a minus lens with high-precision and high-quality, and a spectacle lens produced by using the method.
2. Description of the related art
As a molding method for obtaining a lens from a thermoplastic resin, an injection compression molding method is well-known (e.g.: Japanese Patent Application Laid-open No. Sho 2-26723, Japanese Utility Model Application Publication No. Hei 6-9826 and so on). The spectacle lens, within the range of -4.00 Diopter to -8.00 Diopter of the degree of lens, is preferably used for the molding method.
The injection compression molding method is a method that, in order to obtain uniform high-precise of the configuration by correcting shrinkage of molten resin, the mold is closed to retain a compressing margin in a cavity for lens, and the molten resin is injected to feed into the cavity for lens, and then compressed for the compressing margin to obtain the lens.
Provided that the lens has the different thickness so that the lens is brought about the different thickness when the curvature between the rear face and front face of the lens is different, the lens has two kinds of a plus lens, which the thickness of the central portion is thicker than the peripheral portion, and a minus lens, which the thickness of the central portion is thinner the peripheral portion. Further, the thickness of the lens is different dependent upon a design of an optical lens, for example, an aspherical lens, having approximate 1.50 of index of refraction, can be adjusted to designing values of 4.2 mm of the thickness of the central portion and 1.0 mm of the thickness of the peripheral portion in the case of +2.00 Diopter of the degree of lens, and of 1.4 mm of the thickness of the central portion and 7.9 mm of the thickness of the peripheral portion in the case of -4.00 Diopter of the degree of lens. However, when the above lenses are molded to be compressed under the same conditions, the following disadvantages are produced by the difference of a shape properties.
In the molding of the minus lens, the central portion of the lens is thinner than the peripheral portion thereof, so that the central portion of the cavity has a large flow-resistance. Therefore, the molten resin injected into the cavity hardly flows in the central portion of the cavity, so that the resin flows around from the peripheral portion to the central portion by separately flowing, resulting in a disadvantage that a lot of weld marks are easily produced in the central portion.
Further, in the molding of the plus lens, when the difference between the volume of the cavity and the quantity of the fed molten resin is extremely big, the flow property of the molten resin is reduced, and naturally, a non-fed part is produced, with the result that a flow mark is easily produced on the lens after molding.
It is a object of the present invention to provide an injection compression molding method of a spectacle lens, in which a lens having a different thickness regardless of the shape properties of lens, namely a plus lens and a minus lens, can be molded with a high degree of precision and high quality.
It is another object of the present invention to provide an injection compression molding method of a spectacle lens, in which a plus lens and a minus lens can be efficiently molded from an injecting process to a process for removing from the mold.
It is still object of the present invention to provide a spectacle lens being outstanding for uniform optical properties with a high degree of precision and high quality.